1. Field of the Invention
The present invention broadly relates to the measurement of angular deviation in a transparency and, more particularly, is concerned with an improved two-axis angular deviation measurement system utilizing means for rotating a target image to change between measuring horizontal and vertical angular deviations.
2. Description of the Prior Art
As a general rule, optically transparent, asymmetrically contoured bodies have been difficult to quantitatively evaluate and compare on the basis of their optical characteristics. A prime example of a structural element formed of a transparent medium in which optical quality is critical, yet difficult to quantitatively evaluate, is the canopy or windscreen of aircraft having complex curvilinear contours.
Distortion is one of the optical quality parameters that has been identified for characterizing transparencies, such as aircraft windscreens. Distortion is the non-linear mapping of object points to image space due to the optical effects of the transparency. Such effects may be due to either optical index variations in the transparency or to the opposite faces of the transparency being non-parallel.
In a more technical vein, distortion has been defined as the rate of change of angular deviation across the transparency. Angular deviation is defined as the angular deflection or change of direction of a light ray as it passes through the transparency. Theoretically, the distortion in any transparency may be determined by mapping at a plurality of locations on the transparency the angular deviation of light rays as they are transmitted from the object through the transparency to the observer.
While several methods of measuring angular deviation have been employed in the past, none have proven to be as satisfactory as the system disclosed in the above cross-reference application. This system uses incoherent light and a unique target configuration, and achieves excellent accuracy and repeatability. The unique target configuration is an opaque slide with a transparent pattern in the shape of an "L". Images of the legs of the "L" are used to detect and measure the azimuth (horizontal) and elevation (vertical) components of angular deviation for each tested point on the windscreen.
After the image of the transversely-aligned segments or legs of the L are projected through the test region containing the transparent windscreen, the image is reflected and transmitted by a beam splitter into first and second channels having first and second optical axes. First and second linear detector arrays are located respectively along the first and second optical axes for facilitating detection and measurement of azimuth and elevation components of angular deviation for each tested point on the aircraft windscreen positioned in the test region. The first detector array is located across one of the linear segments or legs of the L, whereas the second detector array is located across the other linear segment or leg of the L. The first and second detector arrays detect lateral shifts in the position of the respective images of the transparent pattern linear segments which correspond to azimuth (horizontal) and elevation (vertical) components of angular deviation for a particular point on the transparency, the lateral shifts occurring if at all, when the transparency is inserted into the test region.
While this system of the cross-referenced application accomplishes its intended purpose in an uncomplicated and efficient manner, it requires rather expensive electro-optical components, such as the CCD components which serve as the arrays. Therefore, a need exists for simplifying the design and reducing the cost of the system without eliminating any of the advantages fostered by the system in carrying out quantitative measurement of angular deviation in transparencies.